What the reactive strength index is

The test is a drop jump: step off a box, land on both feet, and jump straight back up as fast and as high as you can. The measuring system records how high you jumped and how long your feet spent on the ground, and RSI is the first divided by the second. Higher is better, because it means more height out of less ground time. The scale of the number depends entirely on how it is measured, most commonly jump height in metres calculated from flight time, divided by contact time in seconds, so scores only compare when the method and the box height match.

Why it matters for rugby

Sprinting, side stepping, and bouncing back to your feet all reward legs that produce force in a brief touch of the ground, and that short, sharp contact is the quality a drop jump isolates. The number separates playing levels, too: when professional and academy rugby union players went through the same strength testing battery, the professional forwards recorded a reactive strength index around three quarters of a point higher than the academy forwards, a large gap on the scale this test uses.

How the test is run

• Step off a 40 cm box with your hands on your hips. Step, do not jump up or sink down first.
• Land on both feet on the forefoot, then drive straight back up for maximum height with minimum ground contact.
• Stay upright through the jump, with no knee tuck, and land back on the spot you took off from.
• Take three attempts with thirty to sixty seconds of rest between them.
• Measure on a force plate, contact mat, or a phone app that records contact time and flight time. Jump height is calculated from flight time.

What a measured squad scores